1. Field of the Invention
This invention relates to the field of aerial reconnaissance camera systems. Generally speaking, aerial reconnaissance cameras are available in framing and pan scanning configurations. The present invention relates to both types of camera configurations, in that, on the one hand, the step-frame camera system generates individual frames of imagery, while on the other hand, the stepping operation of successive frames of imagery in a cycle generates sweeping scene coverage similar to that obtained with a panoramic camera.
2. Background Art
In a prior art framing camera, an exposure is taken over a large area of fixed format. The field of view of the camera is a function of lens focal length and the geometrical format size of the image recording media. The exposure time is generally controlled by a shutter and is a function of 1) the sensitivity of the photosensitive media, 2) lens transmittance and relative aperture, and 3) available scene brightness. The photosensitive material can be film, an area array Charge Coupled Device (CCD), or any other media which records an image for later retrieval.
Forward Motion Compensation (FMC), also known as Image Motion Compensation (IMC), is a technique used in framing cameras to correct for the image motion on the recording media caused by forward motion of the aircraft during the exposure interval. This correction is generally introduced by moving the film or the lens to keep the image stationary in the fore/aft direction while the exposure is taking place. In a framing camera, the correction is usually accomplished by moving the film to match the rate of image motion.
One limitation of a conventional film or CCD framing camera is that only a single FMC rate can be applied to any given frame regardless of the field of view. Consequently, the motion can exactly be corrected for only a portion of the image. When exposure times are short and the field angles small, this is acceptable. However, for larger fields of view and where longer exposure times are required (as at dusk or under other low light level conditions), the differential rate of motion between the film and the image increases with the field angle and can be large enough result in image blur at the edges of the field. A recent breakthrough in forward motion compensation in electro-optical framing cameras is disclosed in the Lareau et al. patent, U.S. Pat. No. 5,155,597, assigned to the assignee of the present invention. The Lareau et al. '597 patent, which is incorporated by reference herein, describes an electro-optical imaging array that accomplishes FMC electronically and without moving parts by dividing the columns of the array into multiple column groups, and by transferring pixel information in the column groups at a rate that substantially matches the rates of image motion in the column groups.
Another operational function of a framing camera is the generation of an overlap between successive frames of imagery. The overlap is used to ensure complete coverage of all areas of the scene, and/or to provide a view of the scene from two different angular perspectives yielding stereo imagery. In a conventional framing camera, the amount of overlap is selectable and nearly always takes place in the direction of flight.
FIG. 1 depicts a prior art framing camera having a lens 40 of focal length f, film 10 of area W.times.W, and exposure slit 12 which moves at velocity V.sub.S. The film is moved at velocity V.sub.F to avoid image smear due to the forward motion of the aircraft (not shown) at velocity V. Two successive frames of a scene are taken at an interval T.sub.C. The frame time T.sub.F is given by W/V.sub.S. The return time for the shutter to retrace to its initial position is T.sub.R. In FIG. 1, the overlap L(OL) of the two frames of imagery is about 10%, which is generally the minimum overlap. Typically overlap values of 10% or 12% and 56% are made available. An overlap of at least 50% allows all imagery in the adjacent frames to be exposed from two different angular perspectives. These images can be recombined by means of a stereo viewing system to achieve depth perception. Such stereo images are often used by a photointerpreter to gather additional information about the scene.
The operation of a film-type framing camera in a stepping mode is known in the art. For example, the article entitled "The KS-146A LOROP Camera System", Thomas C. Augustyn, SPIE Proceedings Vol.9, Aug. 27-28 1981, paper 309-11 p.76, describes an automatic stepping mode in which the camera cycle rate is proportional to aircraft velocity, altitude and selected depression angle, to achieve 56% overlap for stereo viewing or 12% overlap for maximum flight line coverage. With the camera line of sight normal to the flight path, the scan head provides either 1, 2, 4, or 6 lateral-step cycles. A similar stepping operation for a frame camera is described in the article entitled "KS-127A Long Range Oblique Reconnaissance Camera for RF-4 Aircraft", Richard C. Ruck and Oliver J. Smith, SPIE Proceedings Vol. 242, Jul. 29-30, 1980 San Diego Paper 242-02, p.22. However, the application of a stepping mode of operation to electro-optical framing cameras has, heretofore, eluded those in the art.
Panoramic (pan) camera technology is another well-established means of imaging and provides distinct advantages over the framing camera. In a panoramic scanning camera, as depicted in FIG. 2A, the scene is exposed continuously by rotating a scanning mechanism 14 (such as a double dove prism) so as to scan the image across the photosensitive medium 10. The angle delta The photosensitive medium 10 is moved in synchronism with the image. In the case of a film camera, this may be accomplished by moving the film at a constant rate past an exposure slit 12 which is located on the lens optical axis. A scan prism 14 located in front of the lens 40 is rotated in synchronism with the film rate such that the image of the scene remains stationary on the film during the exposure period. The slit opening is adjusted to a predetermined width to control exposure time.
One major advantage of a pan camera is its ability to image a large area in the direction across the line of flight. Scan angles across the line of flight on the order of 120 to over 180 degrees are typical. See FIG. 3. The lens field of view in a pan camera is generally only required to be large enough to cover the width of the film. Overlapping of images and stereo imagery may also be obtained with pan cameras. FIG. 3 shows overlapped pan camera images and the cycle time computation, with the depression angle delta equal to 90 degrees. Image overlap in a conventional fixed mounted pan camera is obtained as in the case of a framing camera, that is, in the common area between successive scans.
FMC for both the film and electro-optical versions of the pan camera is usually accomplished by a conventional electro-mechanical means. Translating the lens during the scan is a popular means to achieve graded FMC as a function of instantaneous slant range to the scene. This is illustrated in FIG. 2B, where V.sub.L is the lens velocity required for FMC.
In a pan camera system when the photosensitive medium is a linear electro-optical array, read out of the array to a data storage device and/or image display occurs dynamically while the image is being collected. The maximum scan rate which can be achieved is a function of the maximum data rate at which the array may be read out and the data processed. For electro-optical operation, a problem typically arises in that the scan rate of an electro-optical pan camera is very slow because it is limited by the CCD readout rate. Since the forward motion and altitude of the aircraft, expressed as a velocity/height (V/H) ratio, determines the FMC rate of travel, and since this is independent of camera scan rate, the translating lens used for FMC correction quickly runs out of travel when the scan rate is slow, which limits the angular field that can be scanned.
Thus, there has existed a need for an electro-optical imaging array that can obtain broad areas of scene coverage, in the manner of a panning camera, without the above limitations. The present invention provides an electro-optical step-frame camera system which generates high-resolution overlapped scene imagery without blur due to FMC errors. As such, it facilitates the processing and viewing of the digital stereo imagery on a computer or workstation equipped with a stereo monitor having active or passive glasses. The present invention is superior to prior film-type framing and panning cameras because it eliminates the need for a translating lens to accomplish FMC, and allows the camera to be independently repositioned during the stepping cycles during readout of the array. Further, it overcomes the single fixed rate FMC correction prevalent in the conventional framing cameras, producing extremely accurate image motion compensation in every frame of imagery.
The present invention possesses additional advantages and features. The camera system provides greater cross line of flight coverage, and therefore total area coverage, at nearly equivalent resolution to an electro-optical pan camera, even though the data rates of both cameras may be identical. In electro-optical pan cameras, the scene coverage that can be imaged a given time span is constrained by the relatively small number of pixels in a one-dimensional array. In the present invention, a much larger portion of the scene is imaged in a comparable time span, since the array is a two-dimensional array. Moreover, the present step-framing invention permits the readout of data from the array during the camera retrace period, essentially giving an extra picture during the stepping cycle, whereas prior art line scan panning cameras lose the retrace time since the array collects scene information and reads it out at the same time.
Further, imagery from prior art electro-optical pan cameras suffers from an "S" shaped distortion. This distortion arises from the footprint of the scan pattern across the field of interest, projected onto the ground, due to the forward aircraft motion. The present invention permits a stepping in the roll direction and a reposition in the fore/aft direction, thereby correcting any scene distortion due to forward aircraft motion.
Yet another feature and advantage of the present invention not available with conventional frame or pan cameras is the ability to couple a wide field of view with graded sector FMC and at the same time take pictures with a preselected amount of overlap in either the direction of flight or perpendicular to the direction of flight, to achieve total scene coverage. Additionally, a two-axis stepping is made possible, that is, a stepping of the line of sight between successive frames in both azimuth and depression. Only one stepping mechanism (e.g., a mirror or prism) is needed to accomplish the two-axis stepping, as the stepping mechanism is rotated about two axes. A derotation prism or other equivalent optical element eliminates the combined image rotation effects. The two-axis stepping facilitates optional multi-aspect or rapid azimuth coverage with a single point perspective.
These and other features and advantages of the invention will be apparent from the following detailed description of the invention.